1. Field of the Invention
The invention relates to an assembly comprising a fitting and a pilot device having at least two spring-loaded pilot valves which each actuate a dedicated pilot part, for opening the fitting which is opened by pressure relief by means of a control line.
These are fittings which are actuated by their own medium, preferably safety valves for reducing undesirable excess pressures in pressure lines or pressure vessels. The fittings which are actuated by their own medium are opened by a pilot valve when a critical pressure is reached in a pressure vessel or a pressure line and should be closed by the same pilot valve when the pressure falls below a critical level. The pilot valves are spring-loaded. The preferred application area for fittings which are controlled by their own medium in this way is in the primary circuit of nuclear power plants, in particular pressurized-water reactors.
Spring-loaded pilot valves per se are sufficiently well known in the prior at and are already supplied for these purposes by various companies. A safety valve of this type is known, for example, from German patent DE 455 133. In a pilot valve arrangement which operates on the relief principle, the system pressure (pressure in the pressure vessel) operates against a spring and an additional magnetic load and opens a pre-control pilot valve, downstream of which a nonreturn valve is arranged. As a result, a valve disc of this nonreturn valve is lifted counter to the force of a further spring and lifts a nonreturn valve cone out of its valve seat; as a result, the control line of a pressure-vessel safety valve is opened and the pressure in the control volume of the safety valve is relieved. The safety valve opens. It closes again when the system pressure has fallen sufficiently far for the pre-controlled pilot valve and the nonreturn valve to close.
The valve arrangement of a second manufacturer operates on the loading principle. It essentially comprises only a valve with spring and magnetic load (similar to the abovementioned pilot valve) andxe2x80x94as soon as a setpoint has been reachedxe2x80x94passes a mass flow out of the pressure vessel into the control line, which leads to the control volume of the safety valve and presses it open.
The pilot valve produced by a third manufacturer actuates a plurality of valve bodies which are connected to one another and which are used both to pressurize the control volume of the safety valve (closure of the safety valve) and to relieve the pressure on this control volume (opening).
Spring-loaded pilot valves have a relatively high probability, which does not correspond to that of the main fitting, of failing to close. The inability of a spring-loaded pilot valve to close is 10xe2x88x922/demand, i.e. the probability is that the valve will fail to close once in only 100 demands. By contrast, the probability that the main fitting will fail to close is 10xe2x88x924/demand. However, the relatively high probability that the pilot valve will fail to close means that the low probability of the main fitting failing to close has no effect.
Even with a relatively recent internal development of a pilot valve, the probability of the valve failing to close is still not entirely satisfactory. In this device, a pressure-displacement converter is connected to a pilot valve, the pressure-displacement converter being mechanically connected to a filling cone and to a relief cone, and both cones being moved in the same direction. In a first step, the filling cone can be moved onto a seat by a relatively low force, with the result that a pilot line of the pilot valve is blocked. In a second step, only a relatively high force is able to lift the relief cone off its seat, with the result that the control line of the pilot part is connected to a line which is open to atmosphere and the pressure on the safety valve is relieved (i.e. the safety valve is opened).
Theoretical calculations revealed that the probability of pilot valves failing to close can be reduced by connecting a combination of these pilot valves in series. By way of example, connecting two valves in series would lead to a probability of (10xe2x88x922)2/demand, i.e. of 10xe2x88x924 per demand, of failure to close. A closure failure probability of this level is very similar to the closure failure probability of the main fitting.
Motor driven pilot valves which are connected in series are known in the prior art. They are in some cases also used in combination with a primary circuit safety valve. However, since these motor driven pilot valves operate on the open circuit principle, according to the relevant safety regulations they should not be used on their own. For example, German Regulations TRD 421 state that if there are three safety valves protecting a pressure vessel, only one safety valve may have pilot valves which operate on the open circuit principle. The other two safety valves which are present must have pilot devices which operate on the closed circuit principle.
The only known series-connected pilot valves which operate on the closed circuit principle are solenoid valves. However, these valves cannot be used in the primary circuit of a nuclear power plant, since they may open in the event of a voltage loss.
Known spring-loaded pilot valves, as described in the introduction, for example, cannot be connected in series on account of their design. For this reason, spring-loaded pilot valves are currently generally only used in single pilot valve arrangements in the primary circuit, i.e. even if a plurality of valves are provided, ultimately only a single pilot part which applies or relieves the load on the safety valve is present in the control line of the safety valve. The relatively high probability of the valve failing to close is accepted as inevitable.
2. Summary of the Invention
The object of the invention is to provide a spring-loaded tandem pilot valve which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which specifies a pilot device for a fitting which, although it may contain loading springs, nevertheless leads to a low probability of the fitting failing to close.
With the above and other objects in view there is provided, in accordance with the invention, an assembly, comprising:
a fitting communicating with a control line and being opened by pressure relief via the control line;
a pilot device having at least two spring-loaded pilot valves each actuating a dedicated pilot part, for opening the fitting by pressure relief via the control line;
wherein the pilot parts are arranged at the control line and are configured to open the fitting only when all the pilot parts are in a position in which the load on the fitting is relieved, and the pilot parts closing the fitting as soon as a single one of the pilot parts is in a position imposing a load on the fitting.
In an alternative embodiment, there is provided, in accordance with the invention, an assembly, comprising:
a fitting communicating with a control line and being opened by pressure imposed via the control line;
a pilot device having at least two spring-loaded pilot valves each actuating a dedicated pilot part, for opening the fitting by pressure imposed via the control line;
wherein the pilot parts are arranged at the control line and are configured to open the fitting only when all the pilot parts are in a position in which a load is imposed on the fitting, and the pilot parts closing the fitting as soon as a single one of the pilot parts is in a position relieving the load on the fitting.
In other words, the invention specifies a pilot device by means of which a fitting, in order to be opened, can either be relieved or loaded by means of a control line, i.e. the fitting can operate either according to the relief principle or according to the loading principle. The pilot device may also operate without additional electromagnetic or other loads.
To achieve the above and other objects of the invention, the pilot device has at least two spring-loaded pilot valves as well as the same number of pilot parts. Each of the pilot parts is assigned to one of the pilot valves and is connected thereto. According to the invention, the pilot parts are arranged on the control line. A fitting operating on the relief principle opens when, and only when, all of the pilot parts are in a position in which the load on the fitting is relieved. The fitting closes as soon as even only one of the pilot parts is in a situation in which load is applied to the fitting. If the fitting is operating according to the loading principle, it opens when, and only when, the pilot parts are in a position in which they apply a load to the fitting and closes again as soon as even only one of the pilot parts is in a position in which it relieves the load on the fitting.
The invention works on the basis that the probability of the fitting failing to close is reduced in accordance with the rise in number of independently operating pilot parts when each pilot part on its own closes the fitting. A combination of independently acting pilot parts therefore enables the probability of the fitting failing to close to be improved considerably. For example, the combination of two pilot parts with a closure failure probability of in each case 10xe2x88x922/demand results in an overall closure failure probability of (10xe2x88x922)2/demand, i.e. of 10xe2x88x924 per demand. As a result, the failure closure probability of the fitting also corresponds to that of the combined pilot parts.
The pilot parts advantageously contain valves which are actuated by the associated pilot valves and are connected in series in the control line of the fitting. In this case, each of the pilot parts has a first port (the xe2x80x9ccontrol portxe2x80x9d which is connected to the associated pilot valve via a control passage) and two further ports (xe2x80x9cworking portsxe2x80x9d, by means of which the pilot part is integrated into the control line of the fitting). Therefore, a working port of each pilot part is connected to the fitting either directly (for example via a feed line) or via one or more pilot parts (i.e. indirectly). Therefore, only a single pilot part is directly connected to the fitting via a working port, while the other working port of this pilot part is connected to a working port of another pilot part. This arrangement is referred to as a xe2x80x9cseries connectionxe2x80x9d or a xe2x80x9ctandem valvexe2x80x9d, since a pressure medium coming from the fitting or flowing toward the fitting successively passes through a plurality of the pilot parts.
Furthermore, it is considered advantageous that the respective pilot valves and/or pilot parts are of the same design. This restricts fabrication and stock to a few components and makes construction and assembly easier.
In accordance with an added feature of the invention, at least one of the pilot valves of the pilot device has a pressure-displacement converter. This pressure-displacement converter, a filling cone and relief cone are arranged in such a manner that they interact mechanically with one another (at least in some positions are for example, connected to one another) and can be displaced simultaneously in the same direction. In a first chamber, the filling cone is arranged in such a manner that it can be displaced onto a valve seat (xe2x80x9cfilling cone seatxe2x80x9d) by a relatively low force. It is used to block off or open a pressure-carrying connection line, which opens out into the first chamber, with respect to a control passage, which connects this first chamber to the pilot part. The relief cone is arranged in a second chamber and can be lifted off a corresponding valve seat (xe2x80x9crelief cone seatxe2x80x9d) by a relatively great force. As a result of the relief cone being lifted off its seat, a connection is produced between the passage which leads from the pilot part to the first chamber and the volume above the first seat, on the one hand, and a pilot valve drain (e.g. an outlet line which is open to atmosphere) on the other hand.
In accordance with a particularly advantageous feature of the invention, at least one of the pilot valves of the pilot device is designed as proposed in the copending German application 196 28 610.7-12. In this case, the pressure-displacement converter together with the filling cone and the relief cone is arranged on a common central axis. The arrangement is designed in such a way that filling cone and relief cone can be mechanically connected to the pressure-displacement converter and can be displaced in the same direction as the converter (i.e., for example, simultaneously).
The pressure-displacement converter is particularly advantageously provided with a plunger. At its lower end, this plunger is connected to the pressure-displacement converter. The upper end of the plunger is situated beneath and in the immediate vicinity of the relief cone. It can be displaced in such a way that, when suitably displaced, it touches the underside of the relief cone. The plunger is guided in an extension of the filling cone. The second chamber having the seat of the relief cone is situated in the interior of the filling cone.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a spring-loaded tandem control valve, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.